This invention relates to a method of fluidized bed combustion in which heat is generated by the combustion of fuel in a plurality of segmented fluidized beds.
Combustion systems utilizing fluidized beds as the primary source of heat generation are well known. In these arrangements, air is passed through a bed of particulate material, including a fossil fuel such as coal and an adsorbent for the sulfur generated as a result of combustion of the coal, to fluidize the bed and to promote the combustion of the fuel at a relatively low temperatures. When the combustor is utilized as a steam generator, the heat produced by the fluidized bed is utilized to convert water to steam which results in an attractive combination of high heat release, high sulfur absorption, low nitrogen oxides emissions and fuel flexibility.
The most typical fluidized bed combustion system is commonly referred to as a "bubbling" fluidized bed in which a bed of particulate materials is supported by an air distribution plate, to which combustion-supporting air is introduced through a plurality of perforations in the plate, causing the material to expand and take on a suspended, or fluidized, state. The gas velocity is typically two to three times that needed to develop a pressure drop which will support the bed weight (e.g., minimum fluidization velocity), causing the formation of bubbles that rise up through the bed and give it the appearance of a boiling liquid. The bed exhibits a well-defined upper surface, and the entrainment of particles in the gas leaving the bed is quite low, such that collection and recycle of these particles is not always necessary. The heat and mass transfer properties of the two-phase mixture are high, being typical of a liquid.
In a steam generator environment, the walls enclosing the bubbling bed are formed by a plurality of heat transfer tubes, and the heat produced by combustion within the fluidized bed is transferred to water circulating through the tubes. The heat transfer tubes are usually connected to a natural water circulation circuitry, including a steam drum, for separating water from the steam thus formed which is routed to a turbine or to another steam user.
In an effort to extend the improvements in combustion efficiency, pollutant emissions control, and operation turn-down afforded by the bubbling bed, a fluidized bed reactor has been developed utilizing a "circulating" fluidized bed. In these arrangements the mean gas velocity is increased above that for the bubbling bed, so that the bed surface becomes more diffused and the solids entrainment from the bed is increased. According to this process, fluidized bed densities between 5 and 20% volume of solids are attained which is well below the 30% volume of solids typical of the bubbling fluidized bed. The formation of the low density circulating fluidized bed is due to its small particle size and to a high solids throughput, which require high solids recycle. The velocity range of a circulating fluidized bed is between the solids terminal, or free fall, velocity and a velocity beyond which the bed would be converted into a pneumatic transport line.
The high solids circulation required by the circulating fluidized bed makes it insensitive to fuel heat release patterns, thus minimizing the variation of the temperature within the steam generator, and therefore decreasing the nitrogen oxides formation. Also, the high solids loading improves the efficiency of the mechanical device used to separate the gas from the solids for solids recycle. The resulting increase in sulfur adsorbent and fuel residence times reduces the adsorbent and fuel consumption.
However the circulating fluidized bed process is not without problems. For example, during start-up or at lower loads, the entire lower portion of the fluidized bed has to be utilized, notwithstanding the fact that only a portion is actually needed. This requires a relatively long time to complete start-up and, in addition, requires relatively large capacity start-up burners, and relatively large amounts of fluidizing combustion air to maintain an adequate fluidizing velocity. Further, when using the entire lower portion of the bed during start-up and low loads, the temperature of the bed can fall below a value required for stable ignition and it is difficult to achieve relatively quick starts and desired turndown ratios.